The family car will be driven some known distance per year, e.g. 40,000 km. (For an alternative distance, everything changes proportionately.)

The mother will drive 20,000 km; the father will drive 16,000; the son will drive 4,000.

If the mother were to drive all 40,000 and the others 0, or the father to drive all 40,000 and the others 0, the probability of an accident is 1%. If the son were to drive all 40,000 and the others 0, the probability of an accident is 5%.

That's a reasonable interpretation, but the words could also mean:

If the mother were to drive her 20,000 and the others 0, or the father to drive his 16,000 and the others 0, the probability of an accident is 1%. If the son were to drive his 4,000 and the others 0, the probability of an accident is 5%.

Edited to add: the intended meaning is probably the better choice, since the alternative could be stated much more simply as "the probability that the mother in the year is in an accident is 1%; that the father is, is 1%; that the son is, is 5%"

It is quite convoluted, and I wouldn't expect a question
like this one on the exam. It really is a stretch for the
Exam P material. I must have been in a bad
mood when I made it up (or hungover, it was Boxing Day after all) .